WASHINGTON: The United States said President Hamid Karzai could "set the stage" Tuesday for a way out of the Afghan political crisis, after a ruling by a fraud watchdog invalidated his outright election win.

Senior US officials, seeking a reliable partner in Kabul as President Barack Obama wrestles with a fateful decision on whether to deploy thousands more troops, called on Afghan leaders to forge a "legitimate" political process.

Afghanistan's UN-backed Electoral Complaints Commission (ECC) had earlier unleashed fresh political uncertainty by invalidating more than one million suspect votes from 210 polling stations following the August 20 election.

The decision raised the possibility of a run-off election, or for some kind of national unity government between Karzai, facing fierce pressure from the United States and its allies, and his main rival Abdullah Abdullah.

Secretary of State Hillary Clinton, in the latest of a series of signals from Washington, raised the stakes for expected comments by Karzai on Tuesday.

"The ECC has made its findings known and we're looking to hear from President Karzai tomorrow (Tuesday)," Clinton said, saying she was hopeful of a solution in line with "constitutional order" within several days.

"I don't want to preempt in any way President Karzai's statement which will set the stage for how we go forward in the next stage of this."

"He is going to announce his intentions. I am going to let him do that but I am encouraged at the direction that the situation is moving.

Earlier, Obama spokesman Robert Gibbs urged Afghan leaders to follow a legitimate political process out of the electoral logjam.

"It is going to be incredibly important for the world to see that Afghan leaders are willing to make this process legitimate," Gibbs said.

Obama has conducted a string of in-depth classified briefings with top security advisors on Afghan strategy, as he mulls whether to send up to 40,000 more counter-insurgency troops to battle the Taliban.

He is also probing US tactics towards Pakistan, where Al-Qaeda is hiding out in lawless border areas, as the Islamabad government battles its own increasingly bloody insurgency.

Over the weekend, Washington cranked up pressure on Karzai, warning that it could not deploy more than the 68,000 US soldiers expected in Afghanistan by the end of this year without a credible partner in Kabul.

Gibbs did not say whether Washington wanted Karzai to agree to a re-run election or to embrace the idea of a national unity government -- two options raised by key White House advisors over the weekend.

Karzai told UN chief Ban Ki-moon he would "fully respect the constitutional order," a UN spokeswoman, said, while ex-foreign minister Abdullah said a run-off election would restore faith in the democratic process.

Peter Galbraith, a US diplomat sacked by the UN last month after alleging election fraud told AFP the cheating was even greater than reported and in a full recount "they might have found as many as three million fraudulent votes."

Karzai was feted in Washington when he came to power following the US-led ouster of the Taliban in Afghanistan after the September 11 attacks in 2001.

But many US power brokers have grown disillusioned with the Afghan leader, as allegations of corruption have proliferated and the increasingly unpopular war has dragged into its ninth year.

Obama is also facing a ticking domestic clock: the top Republican in the House of Representatives John Boehner on Monday accused the president of procrastinating on a vital national security decision.

"The current political uncertainty should not be used as a pretext for the White House to back away from the counter-insurgency strategy the President announced in March," Boehner said.

Democracy International, an independent US monitor said the ECC ruling meant that neither candidate had won outright.

The ECC said it ordered the Independent Election Commission (IEC) -- the final arbiter of the election results -- "to invalidate a certain percentage of each candidate's votes" and results from 210 polling stations.

The ECC refused to divulge corrected results, but one Western diplomat and a respected election monitor said Karzai's share of the vote had now fallen to about 48 percent -- not enough for outright victory.